Hey Cina
As the matter of fact, i was too early celebrating victory as i didn“t test the ligt with a grounded power outlet. When i did put the light back into the growspace the problem persisted. That made me realise the problem is in the electronics and the ballast in the light. Somehow the "ground" is leaking back and opening up the diodes in the light with just enough bias current to keep them open. Just a very poor design or a bad quality electronics. So there are two things you could do. Either you open up the light and disconnect the green/yellow ground wire from the chassy, insulate the tip and leave it like that. The con of that is now you have a metal chassy light that has no ground connection so i can be potenitally dangerous. The other way is to put a relay switch between the wall socket and the light to make sure the GROUND is disconnected when the light is off, but then again it will connect the ground to the light when it goes on again. I can write you some more details if you need ... but right now i have to start watering the garden ;)
all the best
Vladimir